The invention relates to magnetic resonance (MR) imaging, and more particularly relates to MR imaging for diagnostic medical applications. In its most immediate sense, the invention relates to medical diagnostic MR imaging in which long spin-echo or long gradient spin-echo pulse sequences are used to form an MR image.
To understand the background of the invention, two long pulse sequences will first be described. The effect of spin-spin relaxation (also known as T.sub.2 relaxation) on the MR images produced by these pulse sequences will also be explained.
A spin echo pulse sequence which is available for use on MR machines manufactured by Siemens AG and which is known as "TurboSE" is schematically illustrated in FIG. 1. This sequence is a so-called "spin echo" sequence because each scan commences with a 90.degree. RF pulse followed by a series of 180.degree. RF pulses (which are known as "refocussing pulses"). After each of these refocussing pulses, a signal is read out. As the pulse sequence continues, the phase-encoding gradient is progressively decreased. Thus, after the first refocussing pulse, the phase-encoding gradient starts out at the most positive value (before spin echo E.sub.1). After the next refocussing pulses, the phase-encoding gradient decreases to 0 (before spin echo E.sub.4 which follows the fourth refocussing pulse). At the end of the pulse sequence, the phase-encoding gradient reaches the most negative value (before spin echo E.sub.7, which follows the seventh refocussing pulse).
FIG. 3 schematically illustrates how the spin echo signals produced by this turboSE pulse sequence are assigned to a 256-row data matrix (known as a matrix in "k-space") which is later subjected to Fourier transformation. The raw data matrix is divided into a plurality of segments; the segments are equal in number to the number of signals obtained within a single scan. (In the present instance, there are seven segments, but this is only for purposes of illustration.) During each scan, one row of each segment is acquired. The sequence is repeated until all rows of all segments have been acquired, i.e. until a-complete set of data has been collected. As can be seen in FIG. 3, spin echo E.sub.1 with the highest positive phase-encoding gradient is assigned to one row of the first segment, spin echo E.sub.4 with a zero phase-encoding gradient is assigned to a row in a central segment (the most significant part for the Fourier transformation with respect to signal-to-noise ratio) and spin echo E.sub.7 is assigned to a row in the last segment with the highest negative phase-encoding gradient.
FIG. 1 and FIG. 3 also correlate the magnitude of the induced spin echo signal (FIG. 2) with the pulse sequence which generates it (FIG. 1) and the resulting k-space matrix (FIG. 3). As is known to persons skilled in the art, spin-spin relaxation (otherwise known as T.sub.2 relaxation) causes the induced MR spin echo signal to die out. Therefore, while a long spin echo pulse sequence as is illustrated in FIG. 1 produces a high amplitude signal at the beginning of the sequence (where the phase-encoding is positive), a signal of small amplitude is acquired at the end of the sequence (where the phase-encoding is negative). As a result, the resulting k-space matrix has a signal amplitude which progressively diminishes from segment to segment. When the k-space matrix information (time domain information) is transformed to image information (frequency domain information) during Fourier transformation, this is manifested as a blurring of the MR image, leading to a loss of resolution.
FIG. 4 schematically illustrates another pulse sequence of the type disclosed in U.S. Pat. No. 5,270,654. In this sequence, each refocussing pulse is read out three times in a row by initially using a positive readout gradient, subsequently using a negative readout gradient, and finally using a positive readout gradient. (There is some inconsistancy in the terminology used to describe the resulting triplet of echo signals. It is clear that the middle echo in the triplet is a "spin echo" signal, but some persons have referred to the outer two echos of the triplet as "gradient echos". This is not precisely correct; these echos will hereinafter be simply referred to as "echos".) Furthermore, each one of the echo signals in the triplet is read out at a corresponding phase-encoding gradient; the gradient varies in each instance. Because this is a spin echo sequence wherein there are a plurality of read outs using different gradients after each refocussing pulse, this type of pulse sequence is known as a gradient spin echo sequence.
Thus, in this type of gradient spin echo sequence the first refocussing pulse is followed by three echo signals: E.sub.1, E.sub.2 and E.sub.3. Echo signal E.sub.1 is read out at a maximally positive phase-encoding gradient, echo signal E.sub.2 is read out at a slightly positive phase-encoding gradient and echo signal E.sub.3 is read out at a slightly negative phase-encoding gradient. The same pattern is repeated after each subsequent refocussing pulse, except with phase-encoding gradients of different magnitudes.
In this instance, the information produced by the nine echo signals E.sub.1 through E.sub.9 is reshuffled before it is input into the k-space matrix. This is because the k-space matrix information is organized by the magnitude of the phase-encoding gradient. FIG. 6 shows where the information from the nine echo signals generated by the gradient spin echo pulse sequence appears in the k-space matrix.
As in the previous instance, FIGS. 4, 5 and 6 also show the effect that T.sub.2 relaxation has on the echo signals which are produced by this turboGSE pulse sequence. FIG. 6 clearly shows that the decay in the echo signals which is produced by T.sub.2 relaxation, combined with the reshuffling of the echo signals when they are input to the k-space matrix, produces a periodicity in the k-space matrix; in each group of three adjacent rows in the k-space matrix, the first segment has the largest magnitude, the second segment has a lesser magnitude and the last segment has the least magnitude.
The time-domain information in the k-space matrix of FIG. 6 is converted into image information (frequency-domain information) using Fourier transformation. When this happens, the above-described periodicity in the contents of the k-space matrix is decoded as a so-called "ringing artifact", causing the final MR image to contain faint ghosts and inducing a loss of resolution.
It is therefore one object of the invention to provide a method of acquiring and processing MR data so that spin-spin relaxation does not degrade the reconstructed MR image, even in long pulse sequences.
Another object of the invention is to provide such a method which can be used without unduly prolonging an MR study in which the method is employed.
Still another object is, in general, to improve upon known MR pulse sequences.
In accordance with the invention, pairs of spin echo signals which have identical phase-encoding and which differ in amplitude as a result of spin-spin relaxation (i.e. which occupy different temporal positions in the same sequence or in different sequences) are averaged. The resulting averaged signals are then used in the place of the original spin echo signals. In preferred embodiments, the pairs of spin echo signals can be drawn from a single pulse sequence or from two identical and successive pulse sequences. The sequences may be of the full-Fourier or the half-Fourier type.